feat: add --force-render-cgroup args to make lxcfs render in view of specific cgroup

[okhowang]

it's useful in some complex container hierarchy,
many container work in same parent cgroup for sharing resource (cpu and other)
but need inspect total usage of parent cgroup in single container

[mihalicyn]

Hi @okhowang,

please, could you sign-off your commit in accordance with our contribution guideline (https://github.com/lxc/lxcfs/blob/main/CONTRIBUTING.md).

Also, please, describe your changes and use case. From what I see in the code, you basically force to always use the same cgroup to take data from.

Kind regards,
Alex

[mihalicyn]

I've left a comment before.

[okhowang]

I use --cgroup-parent and --pid to compose multiple container in a unified resource limit and pid namespace, just like a vm.

# setup main.slice cgroup
docker run --cgroup-parent main.slice --name main debian # as main container
docker run --cgroup-parent main.slice --pid container:main --name sidecar debian # as sidecar container 
# and more container

in the pattern containers can see each others, and share same resource.
but if they inspect their resource limit via lxcfs, lxcfs will always render the pid namespace owner's resource (the main containers)

because docker will create a new cgroup in cgroup-parent
e'g main.slice/docker-a.scope main.slice/docker-b.scope and so on.
all containers will get resource view for main.slice/docker-main.scope, because main containers is pid namespace owner.

I want they all got resource view in main.slice